A left-sided defender who also has played the left side of the midfield, Brennan grew up in Newmarket, Ontario, and started playing with Bristol City youth team in 1994 and made his professional debut in 1996 with Bristol City against cross town rivals Bristol Rovers in the Football League First Division. In 5 years Brennan had five managers including Russell Osman, Benny Lennartsson, Joe Jordan, John Ward and Tony Pulis. In 64 first-team appearances for City over five seasons, Brennan scored three goals. Brennan started his career in the Bristol City youth team and worked his way into the first team before being sold to Nottingham Forest.

Brennan joined Nottingham Forest for £1.5 million in October 1999 and was bought by ex-England captain David Platt.[citation needed] Brennan was the first Canadian-born player to be sold over the 1 million pound mark.[citation needed] In 146 games played with Forest over four seasons under David Platt and Paul Hart, Brennan scored just once, in a 4–0 victory against Norwich City,[1] the team he would later go on to sign for. While he was recovering from a double hernia he had a short loan spell at Huddersfield with his old manager Joe Jordan. He also came on as a substitute in two league games while on loan to Huddersfield in 2000–01 and received a red card against Birmingham City and headed back to Forest.[citation needed]

Brennan joined Norwich City on a free Bosman transfer in 2003 and was managed by Nigel Worthington. During the 2003–04 season in Division One of the Football League, Brennan scored twice in just nine appearances, as he battled an abductor muscle injury. His goals came against Everton in the FA Cup[2] and Coventry City in the league.[3] Despite his personal struggles with fitness, the season saw Norwich win the First Division title and promotion back to the Premier League. Brennan became the first Canadian born outfield player to play in the Premier League.

Brennan joined George Burley at Southampton on 27 January 2006 with his contract due to expire in summer having failed to make an impact on the Norwich first team after a series of injuries. However, after finishing his contract he left the club the following May and departed England to play for his hometown team Toronto FC.

On 8 September 2006, Brennan signed with MLS team Toronto FC for the 2007 season, becoming the first player and captain in club history. Brennan remained captain under Mo Johnston, John Carver, Chris Cummins and Preki.[4] He also became the first Canadian to score for Toronto FC, registering a goal off a free kick against the Columbus Crew on 26 May 2007. This goal subsequently became a Sierra Mist Goal of the Week. Brennan went on to play 27 games, all of them starts, for Toronto FC, the most of any player. He earned a reputation as Toronto's "Iron Man," as he managed to play many of his games despite a rib injury, and looked as though he would go on to play every match of the season. Unfortunately, a knee injury prevented Brennan from achieving this feat. Brennan retained the captaincy for the 2008 season, and nearly managed to score in his team's opening match. Brennan has since added goals in 2008 against Chivas USA in Carson, California and to open the 2009 season at Kansas City, off pass from newly acquired Canadian international teammate Dwayne De Rosario.

Brennan also made an appearance during the 2008 MLS All-Star game in his home stadium, BMO Field. The MLS All-Stars won the game 3–2 against West Ham United. On 6 April 2010 Brennan retired as player to become the assistant general manager for Toronto FC.[5]

Following Brennan's retirement Toronto FC named him assistant general manager to Mo Johnston. However following the firing of Johnston in the late 2010 season Brennan's position remained unknown with the club until the new management team was put in place. It included Paul Mariner as Director of Player Development and Aron Winter as head coach.[8] On March 1, 2011 the club announced that the new position of Brennan within the club would be as head coach of TFC Academy U-17 team, this would be Brennan's first coaching position.[9] On 14 May 2012 it was announced that Brennan would become assistant coach to Aron Winter for Toronto's first team with former assistant Bob de Klerk promoted to technical manager.[10] Brennan is also Currently Toronto FC Reserve Team Coach as well as First Team Assistant Coach.[11] Brennan filled in for Ryan Nelsen on September 28, 2013 who was serving a suspension.[12]